1. Field of the Invention
The field of the invention is that of electromagnetic actuators and in particular those forming part of accelerometers intended for geophone applications. Geophones allow very detailed analysis of the nature of the Earth""s strata with a view to detecting any sources of oil. Excitation sources send mechanical waves into the ground and these act on the accelerometer, its response providing information about the nature of the Earth""s strata encountered by the mechanical waves. In such an application the actuator operates with a travel of very small amplitude and must have very high accuracy.
2. Discussion of the Background
An electromagnetic actuator comprises a stationary stator element coupled to a magnet, a coil and a movable polar element. The actuator is intended to displace an external member secured to the movable polar element.
In an accelerometer, the movable external member is a movable mass coupled to a slaving system with means for detecting its position. The slaving system delivers a slaving current to the coil so as to compensate, through electromagnetic induction phenomena, for a displacement of the mass under the effect of external conditions, for example seismic movements.
The value of the acceleration experienced by the mass is deduced from the value of the slaving current flowing through the coil which is necessary to prevent the displacement of the mass.
The electromagnetic actuator can be employed in other applications in which it operates in a similar manner, for example, the external member to be displaced can be a valve needle which the movable polar element must displace from an initial position to a final, working position. Means for detecting the position of the needle or of the movable polar element are provided so as to control the supply to the coil through a slaving current which allows a displacement of the movable polar element through electromagnetic induction. In this application, very high accuracy in the displacements is also required.
FIG. 1 diagrammatically illustrates an exemplary accelerometer incorporating an electromagnetic actuator of known type.
The electromagnetic actuator referenced 10 comprises a stationary stator element 12 comprising a radially magnetized permanent magnet 5. The magnet 5 surrounds a coil 6 inside which there is a movable polar element 14 secured to a mass 2 to be displaced. The stationary stator element 12 and the movable polar element 14 define between them at least two gaps 51 and form a magnetic circuit. The magnet 5 creates magnetic field lines in the magnetic circuit across the gaps. In the example, the stationary stator element 12 is in the form of an enclosure represented as a cylinder of revolution and the magnet is represented as an annulus. In the accelerometer application, the mass 2 is suspended from a peripheral frame 3 by arms 80, 81. The peripheral frame 3 is fixed to the stationary stator element 12. A system 7 for slaving the position of the mass 2 to be displaced and hence the position of the movable polar element 14 generates a slaving current in the coil 6 as long as the mass to be displaced is not in a reference position. This slaving system has the effect of preventing any displacement of the mass 2 under the effect of external conditions, for example seismic movements. More precisely, the slaving system 7 comprises means 82, 83 for detecting the position of the movable mass 2 and hence of the movable polar element 14 and means 70 for causing a slaving current to flow through the coil 6 when the mass leaves a so-called reference position, in such a way as to displace the movable polar element 14 through electromagnetic induction phenomena until the mass 2 reverts to its reference position. The movable polar element 12 compensates for the displacement which the mass 2 undergoes under the effect of the external conditions.
The value of the displacement which the movable mass 2 would have undergone, under the effect of the external conditions, is deduced from the value of the slaving current required to prevent the displacement of the movable mass 2.
In the electromagnetic accelerometer application, the means for detecting the position of the mass may be piezoelectric strain gauges 82, 83 placed on the arms 80, 81 for suspending the mass 2.
When the supply to the coil 6 is off, the movable polar element 14 is subject to various magnetic fields represented by arrows in FIG. 1. These arrows relate to the magnetic fields created by the permanent magnet 5. The polar element 14 is in unstable equilibrium and tends to move along an axis YYxe2x80x2 perpendicular to the mid plane of the magnet 5 and leaves a middle position represented by the axis AAxe2x80x2 and thereby displaces the movable mass 2. In this middle position, the movable polar element 14 is placed symmetrically with respect to the permanent magnet 5.
When the slaving system 7 comes into play and a slaving current flows through the coil 6, it creates a compensating magnetic field (not represented) which tends to return the movable polar element 14 to the mid plane AAxe2x80x2 and hence to return the movable mass 2 to its reference position.
Given the great sensitivity demanded of such accelerometers, which have to detect very small accelerations, the components of the actuator must be perfectly dimensioned and also positioned with respect to the mass, to the peripheral frame and to the strain gauges before commencing a series of measurements.
It is desired that, when the accelerometer is switched on, in the absence of motion, the mass 2 should be in its reference position, this corresponding to a strainfree state for the piezoelectric strain gauges 82, 83. This state corresponds to an xe2x80x9celectrical zeroxe2x80x9d. This condition implies that the base of the stationary stator element 12, which is secured to the peripheral frame 3, and the face of the non-magnetic element which is fixed to the mass 2 should be in one and the same plane to within around a micrometer. This non-magnetic element forms the interface between the movable polar element 14 and the mass 2 since the movable polar element 14 is generally not connected directly to the mass 2. This positioning between the stationary stator element 12 and the non-magnetic element corresponds to a xe2x80x9cmechanical zeroxe2x80x9d. This condition also implies that the movable polar element 14 be in a position of xe2x80x9cmagnetic zeroxe2x80x9d. In this position the vector sum of the forces applied to the movable polar element 14 is zero. If the components of the actuator had ideal dimensions, the magnetic field lines in the actuator would be distributed in a substantially equal manner between the two gaps 51.
In point of fact, the succession of dimensions involved in the construction of the components of the actuator can only ensure an accuracy of 10 micrometers if one wishes to avoid an unintended increase in the cost of construction of these components. It is then appreciated that, even if the components of the actuator are assembled with care, on turning on the accelerometer, in the absence of motion, the mass 2 is not in the xe2x80x9celectrical zeroxe2x80x9d position and that the actuator is not in the xe2x80x9cmechanical zeroxe2x80x9d position even if the movable polar element 14 is in the xe2x80x9cmagnetic zeroxe2x80x9d position. The quality of the measurements performed suffers.
To overcome the problems of dimensioning the components of the actuator and to facilitate their mounting and their positioning, the present invention proposes that a polar element for adjusting the position of the movable polar element with respect to that of the stationary stator element be included in the actuator. An adjustment can then be performed before the electromagnetic actuator is operated.
More precisely, the subject of the invention is an electromagnetic actuator intended to displace an external member, comprising a stationary stator element associated with a movable polar element securable to the external member, the stationary stator element and the movable polar element forming a magnetic circuit and defining between them at least two gaps, a magnet helping to create magnetic field lines in the magnetic circuit, a coil for controlling the displacement of the movable polar element through electromagnetic induction phenomena, characterized in that it furthermore comprises an adjustment polar element which cooperates with the stationary stator element so as to place the movable polar element in an adjusted position with tespect to that of the stationary stator element, by deviating the magnetic field lines.
The adjustment polar element is engageable in an opening of the stationary stator element, the insertion of the adjustment polar element causing a displacement of the movable polar element from a given initial position to the desired adjusted position detected by a position sensor, the movable polar element being closer to the opening in the initial position than in the adjusted position.
The adjustment polar element will preferably be given the form of a plug.
In order to keep the adjustment polar element in place after the adjustment, it can comprise a longitudinally striated portion which cooperates with the opening whose edge is smooth.
In order to manipulate the adjustment polar element in a relatively easy manner, the striated portion can be followed by a conical portion, itself followed by an active portion which enters the first through the opening, the active portion being of smaller diameter than that of the striated portion.
It will be possible to construct the adjustment polar element from a soft ferromagnetic material such as an iron-nickel alloy.
It will be possible to use a differential screw for the engagement of the adjustment polar element in the opening, it allows slow and steady insertion into the opening.
It is advantageous for the position sensor to detect the position of the external member secured to the movable polar element and hence to give the position of the movable polar element, it can be used during adjustment and also when the actuator is operating.
A system for slaving the position of the movable polar element is associated with the position sensor, this system providing the coil with a slaving current.
The invention also relates to an electromagnetic accelerometer comprising a movable mass suspended from a peripheral frame and associated with strain gauges, and which incorporates an abovementioned electromagnetic actuator and in which the external member is the movable mass and the position sensor the strain gauges, the stationary stator polar element being fixed to the peripheral frame.
In the adjusted position of the movable polar element, the movable mass and the peripheral frame are in substantially the same plane.
The present invention also relates to a process for adjusting the position of a movable polar element of an electromagnetic actuator, the electromagnetic actuator comprising a stationary stator element associated with the movable polar element, the stationary stator element and the movable polar element forming a magnetic circuit and defining between them at least two gaps, a magnet helping to create magnetic field lines in the magnetic circuit, a coil for controlling the displacement of the movable polar element through electromagnetic induction phenomena, characterized in that it comprises the following steps:
placement of the movable polar element in an initial position with respect to the stationary stator element,
orientation of the electromagnetic actuator in such a way that a displacement of the movable polar element can occur substantially perpendicularly to gravity,
engagement into an opening of the stationary stator element of a polar element for adjusting the position of the movable polar element with respect to that of the stationary stator element, this engagement, by deviating the magnetic field lines, causing a displacement of the movable polar element until the movable polar element has reached the desired adjusted position, this position being detected by a position sensor, the movable polar element being, in the initial position, nearer to the opening than in the desired adjusted position.